This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-200175, filed Jun. 29, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a transportation vessel for transporting a closed vessel, or a so-called canister, containing radioactive substance that involves heat release, and a method of loading the closed vessel.
2. Description of the Related Art
Highly radioactive substances represented by spent fuels from nuclear reactors are reprocessed in order to recover plutonium or some other useful substances that can be used again as fuels. The highly radioactive substances are contained before they are reprocessed. In this case, the spent fuels are put into canisters in a nuclear power plant, and the canisters are transported, in transportation vessels or so-called transportation casks, by ship or truck to a containing facility. Since the spent fuels are highly radioactive, the transportation casks containing them are expected to have high sealing and shielding properties.
Usually, the canister comprises a tubular metallic vessel body closed at the bottom and a basket located in the vessel body. A plurality of spent fuel assemblies are sealed in the vessel body in a manner such that they are supported by means of the basket. A top opening of the vessel body is closed by means of a primary lid and a secondary lid that are welded together.
In general, a transportation cask comprises an open-topped cask body formed of a metal such as stainless steel or carbon steel and a neutron shield, which is formed of a high-molecular material or synthetic resin, for example, and covers the outer periphery of the vessel body. A top opening of the cask body is closed by a bolted lid.
In transporting the canister in the transportation cask from a power plant to a desired facility, the spent fuel is contained in the canister and then in the transportation cask by the following processes.
First, in a decontamination pit, the empty vessel body of the canister is put into the transportation cask with its upper end open, and preparations are made for fuel loading. The basket is set in advance in the vessel body. Subsequently, the transportation cask, having the vessel body therein, is transferred to a cask loading pit filled with cooling water, and is immersed in the cooling water. Thereupon, the canister and the cask are filled with the cooling water.
In the cask loading pit, the spent fuel assemblies, having so far been contained in a spent fuel pit, are pulled out one after another by means of a pit crane and loaded in succession into the basket in the vessel body. After a given number of spent fuel assemblies are loaded into the vessel body, the shielding plate is fitted into the top opening of the vessel body.
Subsequently, the transportation cask is pulled up from the cask loading pit and transferred to the decontamination pit by means of an overhead traveling crane. In the decontamination pit, a suitable quantity of cooling water is discharged from the vessel body so that the surface of the cooling water in the cask is situated slightly above the spent fuel assemblies, and water in the gap between the canister and the transportation cask is removed.
After the primary lid is welded to the vessel body of the canister to close its opening, in this state, complete dehydration, vacuum drying, inert gas replacement, sealing operation, welding portion inspection, and air leakage inspection are carried out. Further, the secondary lid is welded, and inert gas replacement in the space between the primary and secondary lids, sealing operation, welding portion inspection, and air leakage inspection are carried out. Thus, seal-welding operation for the lids of the canister is finished, whereupon the canister is completed contained the spent fuel.
Thereafter, the top opening of the transportation cask is closed by means of a lid, and a pre-transportation check is conducted, whereupon pre-shipment preparations are completed. Then, the transportation cask, thus containing the canister, is transported from a power plant to a containing facility.
In loading the spent fuel assemblies into the canister, in the transportation cask constructed in this manner, the canister surface and the inner surface of the transportation cask are also brought into contact with the cooling water and contaminated with radioactivity. In any stage before the transportation, therefore, the canister must be loaded again into the transportation cask after it is temporarily drawn out of the cask to have its outer surface washed and checked for contamination.
Thus, the operation for loading the canister into the transportation cask is very troublesome. In raising the canister from or loading it into the transportation cask, moreover, there is a possibility that the canister will fall by mistake, resulting in damage to the canister and the transportation cask.
Usually, the primary and secondary lids of the canister are checked for the welding state by an ultrasonic sensor or the like after they are welded. In this case, however, the canister is contained in the transportation cask, and the gap between the outer surface of the canister and the inner surface of the transportation cask is narrow. Accordingly, the welding portion can be inspected only from above the canister, so that it is hard to check up on the welding state accurately.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a transportation vessel, capable of preventing contamination of the outer surface of a canister as a radioactive substance and the canister are loaded and simplifying the loading operation, and a loading method for a closed vessel.
In order to achieve the above object, a transportation vessel according to an aspect of the invention is a vessel for contained and transporting a closed vessel, which comprises a substantially tubular metallic vessel body, having a closed bottom and a top opening and configured to contain radioactive substance, and a lid welded to the vessel body and closing the top opening of the vessel body. The transportation vessel comprises: a body having a top opening and provided inside with a containing portion configured to contain the closed vessel; a lid closing the top opening of the body; and a ring-shaped seal member provided between the outer surface of the closed vessel and the inner surface of the body, near the top opening, and configured to seal the gap between the outer surface of the closed vessel and the inner surface of the body and to prevent a fluid from getting into the gap between the outer and inner surfaces through the top opening.
A method of loading a closed vessel with a radioactive substance into a transportation vessel, according to another aspect of the invention, comprises: locating an empty vessel body with an opened top of the closed vessel in the containing portion of an open-topped body of the transportation vessel; sealing the gap between the outer surface of the vessel body and the inner surface of the body by means of a ring-shaped seal member provided between the outer surface of the vessel body and the inner surface of the body, near the top opening of the body, thereby preventing a fluid from getting into the gap between the vessel body and the inner surface of the body through the top opening; immersing the body containing the vessel body in water in a state such that the gap between the outer surface of the vessel body and the inner surface of the body is sealed by the seal member; loading a radioactive substance into the vessel body in the water; setting a shield member in the top opening of the vessel body in the water after loading the radioactive substance; pulling up the body from the water after setting the shield member and then discharging a given quantity of water from the vessel body and the body; and welding the lid to the inner surface of the vessel body after discharging the water.
According to the transportation vessel constructed in this manner and the loading method, the fluid can be prevented from getting into the gap between the vessel body of the closed vessel and the inner surface of the body through the top opening of the body by sealing the gap between the vessel body and the inner surface of the body by means of the ring-shaped seal member that is provided between the outer surface of the vessel body and the inner surface of the body near the top opening. Thus, in immersing the body, containing the vessel body of the closed vessel, in the water to set the radioactive substance in position, the water can be prevented from flowing into the gap between the vessel body and the inner surface of the body through the top opening of the body. In consequence, the outer surface of the vessel body can be prevented from being contaminated with the water. Accordingly, the closed vessel can be washed without being pulled up from the body of the transportation vessel. Thus, there may be provided a transportation vessel and a closed vessel loading method such that the radioactive substance and the closed vessel can be loaded with ease.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.